Allow/disallow style selection based on usergroup membership

I'd like to suggest a link between styles and usergroups so we can selectively revoke style choices for guests, or allow premium members access to specific styles, etc.

In particular I want to present a non-responsive only style to guests (no ability to select alternative styles), whilst at the same time allow registered members to use a responsive style if they wish.

At present all 'active' styles are selectable regardless of usergroup / guest status.